Heating apparatus

ABSTRACT

This invention relates to heating apparatus for imparting heat to a fluid. The heating apparatus comprises combustion chamber means, and burner means at one side of the combustion chamber means. The burner means incorporates a plurality of inlet orifices through which a fuel/air mixture can enter the combustion chamber means, the heating apparatus having a plurality of outlet passages through which the products of combustion of the mixture when ignited can leave the combustion chamber means. The outlet passages are formed within a core of heat conducting material forming at least a part of a heat exchanger. The burner means and the combustion chamber means are arranged so that flames formed by ignition of the fuel/air mixture will be held at positions from which the major portion of the heat arising from combustion of the fuel/air mixture will be conducted through the core to the fluid to be heated.

United States Patent 1191 Ryffin-Hughes et a1.

1 51 May 7,1974

[ HEATING APPARATUS [76] Inventors: John Malcolm Ryffin-Hughes, 27

Witherford Croft, Solihull, Warwickshire; Brian Harding, 28 Arlington Rd., West Bronwich Staffs; Graham Albert John Lake, 30 Green Acres, Birmingham, all of England [22] Filed: Aug. 30, 1971 [21] Appl. No.: 176,229

[30] Foreign Application Priority Data Morck, Jr. I 431/158 Nomaguchi et a1 431/285 2,379,017 6/1945 McCollum 126/118 x FOREIGN PATENTS OR APPLICATIONS 623,728 1/1936 Germany 431/157 Primary Examiner-Carroll B. Dority, Jr. Assistant Examiner-Harold Joyce I Attorney, Agent, or FirmHolman & Stern [5 7] ABSTRACT This invention relates to heating apparatus for imparting heat to a fluid. The heating apparatus comprises combustion chamber means, and burner means at one side of the combustion chamber means. The burner means incorporates a plurality of inlet orifices through which a fuel/air mixture can enter the combustion chamber means, the heating apparatus having a plural-' ity of outlet passages through which the products of combustion of the mixture when ignited can leave the combustion chamber means. The outlet passages are formed within a core of heat conducting material forming at least a part of a heat exchanger. The burner means and the combustion chamber means are arranged so that flames formed by ignition of the fuel/air mixture will be held at positions from which the major portion of the heat arising from combustion of the fuel/air mixture will be conducted through the core to the fluid to be heated.

5 Claims, 5 Drawing Figures slaoslosz EATENTEU MAY 7 1974 saw 3 0F 3 HEATING APPARATUS This invention relates to heating apparatus and has as its object the provision of such apparatus in an improved form.

In accordance with the present invention there is provided heating apparatus for imparting heat to a'fluid comprising combustion chamber means, burner means at one side of the combustion chamber means, said burner means incorporating a plurality of inlet orifices through which a fuel/air mixture can enter the combustion chamber means, said heating apparatus having a plurality of outlet passages through which the products of combustion of said mixture when ignited can leave the combustion chamber means, said passages being formed within a core of heat conducting material forming at least a part of a heat exchanger, said burner means and said combustion chamber means being arranged so that flames formed by ignition of said fuel/air mixture will be held at positions from which the major portion of the heat arising from combustion of said fuel/air mixture will be conducted through said core to said fluid. v

Preferably said burner means and outlet passages are arranged so that the flow of the products of combustion through said outlet passages will inhibit or minimise the formation of stagnant boundary layers within said outlet passages by utilising the expansion of said fuel/air mixture, consequent upon combustion thereof.

Such stagnant boundary layers are normally formed when hot gases pass over a surface to which heat is to be imparted. These layers consist of gases which flow over said surface at a much reduced velocity (such reduction in velocity being caused by frictional effects) relative to the velocity of flow of the remainder of the gases flowing past the surface and this reduced velocity causes said layer to impart its heat to the surface fairly rapidly. Consequently, said layer will become relatively cool in comparision to the remainder of the hot gases. This cool layer serves as a heat insulator between said surface and the remainder of the hot gases thus substantially reducing the efficiency of heat transfer to the surface. Hitherto, attempts have been made to mini.- mise the stagnant boundary layer by complicated mechanical shutter arrangements or by ejecting hot gases at the surface in a direction transverse to said surface. However, the present invention inhibits or minimises this stagnant boundary layer by utilising the natural expansion of gas in a confined combustion chamber and by providing outlet passages of such dimensions that the velocity and turbulence of hot gases through these passages creates a scrubbing action which will disturb or prevent the formation of such layers.

Desirably, the air of said fuel/air mixture includes all the air necessary for combustion of the fuel and preferably the core is formed of a material having a high thermal conductivity.

Preferably said outlet passages correspond in number to the number of inlet orifices in said burner means and desirably each inlet orifice is bounded by two portions of material which are spaced apart by a distance equal to the distance by which corresponding portions of material bounding each outlet passage are spaced apart. Conveniently said inlet orifices and said outlet passages are disposed within a unit formed from two identical extruded blocks of heat conducting material arranged together in face to face relationship.

Alternatively, said heating apparatus may comprise a plurality of co-axial, axially spaced, parallel, annular plates, each of which is provided with a plurality of elongated slots proximate the outer peripheral edge of each plate and extending in a direction parallel to the adjacent edge of said plate whereby first portions of the plates which are disposed outwardly of the slots bound said inlet orifices and second portions of said plates which are disposed inwardly of the slots bound said outlet passages, said combustion chamber means being formed by the slots and the regions between adjacent slots.

Conveniently, said heating apparatus is provided with heat emitting surface means arranged to transfer heat to a fluid which is caused to flow over said heat emitting surface means. Alternatively, a plurality of tubes for the reception of liquid to be heated extend through said core.

The invention will now be more particularly described with reference to the accompanying drawings wherein: p

FIG. 1 is an exploded view of one embodiment of I heating apparatus constructed in accordance with the present invention,

FIG. 2 is a sectional view of a further embodiment of heating apparatus constructed in accordance with the present invention,

FIG. 3 is a sectional view taken along the line 3-3 of FIG. 2, j I

FIG. 4 is a part sectional view of yet a further embodiment of heating apparatus constructed in accordance with the present invention and intended for use as an immersion heater, and

FIG. 5 is a part sectional view of another embodiment of heating apparatus constructed in accordance with the present invention.

Referring now more particularly to FIG. 1 of the accompanying drawings, the heating apparatus shown therein comprises a core 10 which is formed by clamping two blocks 11 and 12 of heat conducting material, and preferably material possessing a high thermal conductivity, such as aluminium, in face to face relationship. Each block 11,12 of material is initially formed by an extruding process such that it possesses a plurality of spaced parallel grooves which are disposed in one face of the block and which each extend between two opposed edges of the block. Each block is then machined, such as by a milling operation, to remove material from said one face of the block such that an elongated recess l3 is formed. The longitudinal axis of the recess 13 is perpendicular to the longitudinal axes of the grooves and this recess divides the grooves into two sets of grooves and at the same time communicates with adjacent ends of the two sets of grooves. Furthermore, the depth of the recess 13 is made greater than the depth of the grooves.

The two blocks 11 and 12 are identically formed and then secured together in face to face relationship. Each groove of each set of one of the blocks 1 1, l2 communicates with a corresponding groove formed in the other block l2, 11. Thus, the two recesses 13 formed in the two blocks 11 and 12 respectively form a combustion chamber 14. One of the sets of grooves in the blocks 11 and the corresponding set of grooves in the other block 12 define burner means inlet orifices 15 whilst the other set of grooves, in the block 1 1 together with the corresponding set of grooves in the other block 12 form a plurality of outlet passages 16 through opposite side with the outlet passages 16. Moreover,

the dimensions of each burner means inlet orifice measured in a direction at right angles to the direction of fuel/air mixture entering these orifices will be equal to the dimensions of each outlet passage measured in a direction at right angles to the direction of flow of the products of combustion.

The exterior of the core is provided with heat emitting surface means in the form of a plurality of fins 17 which are arranged to transfer heat to a fluid (such as air) which is caused to flow over the fins 17.

The two blocks 11 and 12 are clamped together by a pair of elongated plates 18 and 19 each of which is of generally U-shaped cross-section. Each plate 18, 19 comprises two parallel flanges 20 and 21 connected together by an integral web 22. The elongated free edges of each flange of each plate are provided with tapered portions 23 which are engageable with tapered portions 24 of the blocks 11 and 12. The portions 23 are maintained in engagement with the portions 24 by a pair of generally U-shaped stirrups 25 which are formed with an external screw-thread at the free end of each arm of each stirrup. The stirrups engage the external surface of the plate 18 and the screw-threaded ends of said arms of each stirrup pass through apertures 26 formed in plate 27 which engages the external surface of the plate 19. Nuts 28 are then applied to said screwthreaded ends to complete the assembly of the blocks '11 and 12 and of the plates 18 and 19.

The plate 19 serves to define an inlet manifold for the fuel/air mixtureand communicates with the inlet orifaces 15. The plate 18 serves as an exhaust manifold and communicates with the outlet passages 16, and if desired the products of combustion may be diluted in this exhaust manifold. A pair of end plates 29 and 30 are then secured to the aforesaid assembly by bolts which pass through apertures in the end plates and engage in screw-threaded apertures formed in the blocks 11 and 12 and also in a pair of screw-threaded apertures formed in generally U-shaped attachment members 31 which are slidably engaged in slots 32 formed in the external surface of the plates 18 and 19. The plate 30 is provided with an aperture 33 which communicates with the aforesaid inlet manifold and which receives a conduit (not shown) through which the fuel/air mixture is supplied. This plate 30 is also provided with a further aperture 34 which communicates with the combustion chamber 14 and which sealingly receives a suitable high tension igniter 35. The plate 29 is provided with an aperture 36 which communicates with the aforesaid exhaust manifold and which receives an exhaust duct (not shown).

Thus, in use, a fuel/air mixture preferably in the form of a gas/air mixture is fed into the inlet manifold from where it passes through the inlet orifices to the combustion chamber 14. This mixture is then ignited within the combustion chamber 14 by means of a spark produced by the high tension igniter 35. The resulting flames will be anchored at positions where the burner means inlet orifices 15 communicate with the combustion chamber 14 if the conditions are arranged so that the speed of propogation of the flame within the gas/ air mixture is less than the velocity of flow through the inlet orifices 15 and greater than the velocity of flow in the combustion chamber. These resulting flames will be wholly maintained within the heating apparatus. The majority of heat released by combustion will therefore be transferred to the material surrounding the outlet passages and this heat will be transferred through the material of the core 10 to the fins l7 whereupon it will be imparted to air passing the fins. It is to be appreciated that the core 10 is a sealed unit and no secondary air is supplied to the combustion chamber. Thus the fuel/air mixture entering the combustion chamber 14 through the inlet orifices 15 includes at least the whole of the air for combustion.

The above embodiment may be used in a warm air domestic central heating system but it will be appreciated that it has many other uses. Furthermore, by not providing the fins 17 and by increasing the thickness of the blocks 11 and 12, apertures may be formed in these blocks for the reception of conduits through which a liquid, such as water to be heated can flow.

Referring now more particularly to FIGS. 2 and 3 of the accompanying drawings there is shown therein a further embodiment of heating apparatus constructed in accordance with the invention and in this embodiment the core 37 which is formed of heat conducting material is constructed from a plurality of laminations 38 which are each of annular form and which are spaced apart by spacers 39 arranged at intervals around the circumference of the stack of laminations. Thus, each lamination 39 may be of circular or non-circular form, the arrangement being suchthat each lamination will be provided around its circumference with a plurality of apertures 40 of similar size. The spacers are each likewise provided with an aperture which is the same size as the aperture in the laminations. A stack of lami nations is then built up with spacers between each adjacent pair of laminations so that there will be a plurality of apertures extending through the assembly, each of said apertures being formed from a series of aligned apertures formed in the laminations and spacers. A plurality of water carrying conduits 41 are provided to extendthrough said apertures formed in the core and around the-exterior periphery of the core is arranged a burner 42 which may have for example a generally cylindrical piece of apertured metal or alternatively a generally cylindrical piece of metallic mesh which will define the outer boundary of a combustion chamber 43. The apertures in the cylindrical piece of metal or metallic mesh will form inlet orifices 44 through which a fuel/air mixture including at least the whole of the air for combustion can enter thecombustion chamber 43. Thus, the burner is disposed around the exterior of the core 37 and a fuel/air mixture, such as a gaseous fuel- Iair mixture can be lead to the burner inlet orifices 44 through a tube or pipe 45 which extends at one end through the interior of the annular core 37. The heating apparatus of this embodiment is mounted within a housing 46 having an aperture 47 through which the tube 45 enters the housing 46. The aperture 47 is arranged to be of greater dimensions than the external dimensions of the tube 45 for a purpose as will become apparent hereinafter. The aforesaid one end of the tube 45 is flared at 480 so that the fuel/air mixture is then lead round the exterior of the core and into the burner inlet orifices 44. Any suitable ignition means, such as an electrically operated igniter (not shown) is provided in the combustion chamber 43 and the resulting flames will be anchored as described in connection with the first embodiment at the boundary of the inlet orifices 44 and the combustion chamber 43. The products of combustion will pass through outlet passages 48 defined in the core 37 between the laminations 38 and the spacers 39. The products of combustion will then pass through the opening defined between the aperture 47 and the tube 45 whereafter they can be collected in an exhaust duct (not shown).

The water carrying conduits 40 may be connected in series by generally U-shaped connecting tubes 49.

In a modification of this latter embodiment, the core 37 may be constructed from a plurality of similar generally cuboidal units of laminated form. To this end each unit has a plurality of generally rectangular plates which are separated at each end by relatively short spacer pieces so that a pair of said spacer pieces are disposed between each pair of adjacent rectangular plates, the spacers being located respectively adjacent the two ends of the unit. The core may thus be formed by connecting together these units in an end to end relationship so that they form a closed ring or polygon.

Preferably the cores of all the above embodiments are assembled in laminated form such that each outlet passage is bounded on each of two opposite sides by a portion of the core having a thickness which is not less than the width of the outlet passage as measured by the distance between the two core portions defining the outlet passage. Moreover, it will be appreciated with an annular embodiment that the burner will fire radially inwardly towards the core. Furthermore, the majority of heat resulting from combustion will be transferred to the laminations and spacers and thence to the interior of the conduits 41 whereby said interiors will act as heat emitting surfaces from which heat can be transferred to a fluid such as water flowing through the conduits.

In yet a further modification of the embodiments shown in FIGS. 2 and 3, the heating apparatus may comprise a plurality of co-axial, axially spaced annular plates (not shown) of circular form each of which is providedwith a plurality of circumferentially spaced circumferentially extending slots arranged proximate the outer peripheral edge of the plate. The conduits 41 are arranged to extend through apertures formed in the annular plates proximate the inner peripheral edge thereof and these tubes extend in a direction parallel to the axes of the plate. The aforesaid plates may conveniently be spaced apart by spacing elements in the form of sleeves mounted on the conduits 41. Burner inlet orifices will thus be founded by first portions of the plates, said first portions being those portions of the plates disposed radially outwardly of the slots. Outlet passages will be bounded by second portions of the plates, said second portions being those portions of the plates disposed radially inwardly of the slots, and the slots themselves together with the regions between the plates adjacent said slots will form the combustion chamber. Since the discs are arranged in co-axial, axially spaced parallel relationship then each burner inlet orifice is bounded by two portions of material which are spaced apart by a distance equal to the distance by which corresponding portions of the material bounding each outlet passage are spaced apart,

Referring now more particularly to FIG. 4 of the accompanying drawings, the heating apparatus shown therein is intended for use as an immersion heater and it comprises a core 50 of generally cuboidal configuration and of laminated form, the laminations being made of a material which is a good conductor of heat such as aluminium. The laminated core is arranged so that slots are defined with the core, said slots forming outlet passages 51. Preferably, each of said outlet passages have a width (as measured by the distance between a pair of laminationswhich define the outlet passage) which is not greater than the thickness of each of the two laminations. A burner 52 is provided on one side of the core 50, the burner being spaced from the core 50 so as to define a combustion chamber 53 between the core 50 and the burner 52. The burner 52 is in the form of an apertured metal plate wherein the apertures form inlet orifices 54 in the burner. The core 50 and the burner 52 are then arranged within a housing 55 having a pair of opposed tapered walls 56 and 57. The wall 56 and the burner 52 define between them a chamber 58 into which a fuel/air mixture is in usefed. The wall 57 and the core 50 define between them a chamber 59 which communicates with the end of the outlet passages 51 remote from the combustion chamber 53 and it is through this chamber 59 that the products of combustion are exhausted. A fuel/air inlet conduit (not shown) and an exhaust conduit (not shown) are connected to the housing 55 at the end thereof at which the walls 56 and 57 are at a maximum distance apart so as to communicate respectively with the chamber 58 and the chamber 59. The heating apparatus is thus formed as a sealed unit and an electrically operated igniter 60 is arranged to extend into the combustion chamber 53 for the purpose of igniting the fuel/air mixture.

The heating apparatus as above described is then disposed within a chamber (not shown) which may be ar ranged so that a liquid such as water is caused in use to flow through said chamber or which may be adapted to receive the liquid which, from time to time, will be drawn off from the chamber. The exterior of the heating apparatus may be provided with a plurality of fins and may also be formed of or coated with material such as copper which is a good conductor of heat but which is adapted to resist the effects of galvanic corrosion. The heating apparatus will thus be immersed in the liquid in use so that heat imparted to the core by combustion of the'fuel/air mixture will be transferred from the core 50 to the liquid which surround the heating apparatus. Such heating apparatus may be used in a domestic central heating system or in any other application in which it is desired to transfer heat to water or another liquid.

Referring now more particularly to FIG. 5 there is shown therein a core which is formed from a block 61 (which may be cylindrical) of aluminium or other suitable heat conducting material. Extending axially through said block is a conduit in the form of a tube 62 which is arranged so that a fluid (such as water) to which heat is to be transferred can flow through the tube 62. The block 61 is also formed with a plurality of passages 63 which each extend through the block in a direction parallel to the axis of the block. Each passage 63 comprises two portions, namely one portion 64 of cylindrical shape and having a relatively large diameter together with a further portion 65 also of cylindrical shape but having a relatively small diameter. There is thus provided between the ends of each of said passages a step or shoulder 66 formed between the junctions of two aforementioned portions.

One axial face 67 of the block of aluminium is arranged to be connected to a supply of a fuel/air mixture such as a gaseous fuel/air mixture and in one arrangement said one axial face of the cylindrical block may form the upper boundary of a chamber (not shown) of generally conical configuration to which the fuel/air mixture is fed, the arrangement being such that the fuel/air mixture entering the passages in the block 61 from the conical chamber will flow firstly through those portions 65 of said passages of relatively small diameter. Furthermore, the diameters of said portions of the passages in the cylindrical block are so arranged in relation to the pressure of fuel/air mixture supplied and any other relevant parameters such as, for example ambient temperature, that when the fuel/air'mixture flowing'through said passages is ignited a plurality of separate flames will be produced in said passages with the flames anchored on the steps or shoulders 66 between the two portions of each passage. Thus it will be appreciated that owing to the difference in diameters of the two portions 64, 65 of each passage, the-fuel/air mixture will flow at a faster velocity through the portion of relatively small diameter (forming an inlet orifice), than through the portion of relatively wide diameter (forming a combustion chamber and an outlet passage). If the conditions are arranged that the speed of propogation of the flame within the fuel/air mixture is greater than the velocity of flow through the portion of each passage of greater diameter then all the flames will be anchored as aforesaid on the steps or shoulders in said passages. Heat from the flames will thus be conducted directly into the interior of the aluminium block 61 to the tube 62 extending axially through the block and thence to the water or other fluid flowing through said tube.

It is however to be appreciated that the passages 63 may be of tapered configuration or alternatively of cylindrical configuration if the rate of flow of fuel/air mixture to the passages is finely controlled.

Thus, in all the above described embodiments, the burner means is so constructed that in use the flames issuing therefrom will be anchored at a position or positions in which the majority of the heat released by combustion will be transferred to the fluid which is to be heated. Such flames will of course be anchored at positions where the speed of propogation of a flame in the particular fuel/air mixture used and in the particular conditions prevailing is less than the speed of fuel/air mixture flow forwardly at such positions. In this way the heat will be generated'at positions where a majority of the heat can be transferred by conduction of the material defining the outlet passages of the products of combustion and thence to the fluid which is heated and by convection from that surface of the material which is in contact with the fluid. Furthermore, as hereinbefore described, the hitherto formed stagnant boundary layers are inhibited or minimised in all the above described embodiments thus resulting in a substantial increase in efficiency of heat transfer between the products of combustion and the material defining the outlet passages. The efficiency of heat transfer will also be assisted by turbulance in the combustion zones and at the same time the maximum advantage will be taken of radiation off the flames. Furthermore, it will be observed that the fuel/air mixture supplied to the burner means includes the whole of the air for combustion and thus no secondary air is required. In addition the design of the burner relative to the combustion chamber is such that the flames resulting from combustion wull be wholly maintained within the apparatus.

Finally, it is important to realise that in use the heat input to the heating apparatus is arranged to be transferred to said heat emitting surfaces via conducting paths which ensurethat heat flows to the fluid to be heated at a safe and efficient temperature relative to the thermal characteristics of said fluid and said core material.

We claim:

1. Heating apparatus for imparting heat to a fluid, comprising combustion chamber means; burner means at one side of the combustion chamber means, said burner means incorporating a plurality of inlet orifices through which a fuel/air mixture can enter the combustion chamber means; and a plurality of outlet passages through which the products of combustion of said mixture when ignited can leave the combustion chamber means, said outlet passages corresponding in number to .the number of inlet orifices in said burner means and each outlet passage being aligned with one inlet orifice,

said outlet passages and inlet orifices being formed within blocks of heat conducting material, and said burner means and said combustion chamber means being arranged so that flames formed by ignition of said fuel/air mixture will be held at positions from which the major portion of heat arising'from combustion of said fuel/air mixture will be conducted through said core to said fluid.

2. Heating apparatus for imparting heat to a fluid comprising:

combustion chamber means; I burner means at'one side of said combustion chamber means, said burner means incorporating a plurality of inlet orifices through which a fuel/air mix ture can enter said combustion chamber means; a plurality of outlet passages throug which the products of combustion of said mixture when ignited can leave said combustion chamber means, said outlet passages corresponding in number to the number of said inlet orifices in said burner means, and said inlet orifices being bounded by two portions of material which are spaced apart by a distance equal to the distance by which corresponding portions of material bounding each outlet passage are spaced apart, said combustion chamber means and said outlet passages being defined entirely by a core of heat conducting material, said burner means and said combustion chamber means being arrangedso that flames formed by ignition of said fuel/air mixture will be held at positions from which the major portion of the heat arising from combustion of said fuel/air mixture will be conducted through said core to said fluid. 3. Heating apparatus for imparting heat to a fluid comprising:

combustion chamber means; burner means at one side of said combustion chamber means, said burner means incorporating a plurality of inlet orifices through which a fuel/air mixture can enter said combustion chamber means; a plurality of outlet passages through which the product of combustion of said mixture when ignited can leave said combustion chamber means, said outlet passages corresponding in number to the number of said plurality of inlet orifices in said burner means and said inlet orifices and said outlet passages being disposed within a unit formed from two identical extruded blocks of heat conducting material each extruded block having a plurality of grooves therein such that when the blocks are arranged together in face to face relationship the grooves of each block define together said inlet orifices and said outlet passages, said combustion chamber means and said outlet passages being defined entirely by a core of heat conducting material, said burner means and said combustion chamber means being arranged so that flames formed by ignition of said fuel/air mixture will be held at positions from which the major portion of the heat arising from combustion of said fuel/air mixture will be conducted through said core to said fluid.

4. Heating apparatus as claimed in claim 3 wherein said combustion chamber means is formed by removing material from each block to form a recess, said recess having a depth greater than the depth of said grooves.

5. Heating apparatus for imparting heat to a fluid comprising:

combustion chamber means;

burner means at one side of said combustion chamber means, said burner means incorporatinga plurality of inlet orifices through which a fuel/air mixture can enter said combustion chamber means;

a plurality of outlet passages through which the products of combustion of said mixture when ignited can leave said combustion chamber means, said plurality of outlet passages corresponding in number to the number of said inlet orifices in said burner means, the dimensions of each of said inlet orifices measured at right angles to the direction of flow of said fuel/air mixture being equal to'the dimensions of each of said outlet passages measured in a direction at right angles to the direction of flow of said product of combustion, said combustion chamber means and said outlet passages being defined entirely by a core of heat conducting material, said burner means and said combustion chamber means being arranged so that flames formed by ignition of said fuel/air mixture will be held at positions fromwhich the major portion of the heat arising from combustion of said fuel/air mixture will be conducted through said core to said fluid. 

1. Heating apparatus for imparting heat to a fluid, comprising combustion chamber means; burner means at one side of the combustion chamber means, said burner means incorporating a plurality of inlet orifices through which a fuel/air mixture can enter the combustion chamber means; and a plurality of outlet passages through which the products of combustion of said mixture when ignited can leave the combustion chamber means, said outlet passages corresponding in number to the number of inlet orifices in said burner means and each outlet passage being aligned with one inlet orifice, said outlet passages and inlet orifices being formed within blocks of heat conducting material, and said burner means and said combustion chamber means being arranged so that flames formed by ignition of said fuel/air mixture will be held at positions from which the major portion of heat arising from combustion of said fuel/air mixture will be conducted through said core to said fluid.
 2. Heating apparatus for imparting heat to a fluid comprising: combustion chamber means; burner means at one side of said combustion chamber means, said burner means incorporating a plurality of inlet orifices through which a fuel/air mixture can enter said combustion chamber means; a plurality of outlet passages throug which the products of combustion of said mixture when ignited can leave said combustion chamber means, said outlet passages corresponding in number to the number of said inlet orifices in said burner means, and said inlet orifices being bounded by two portions of material which are spaced apart by a distance equal to the distance by which corresponding portions of material bounding each outlet passage are spaced apart, said combustion chamber means and said outlet passages being defined entirely by a core of heat conducting material, said burner means and said combustion chamber means being arranged so that flames formed by ignition of said fuel/air mixture will be held at positions from which the major portion of the heat arising from combustion of said fuel/air mixture will be conducted through said core to said fluid.
 3. Heating apparatus for imparting heat to a fluid comprising: combustion chamber means; burner means at one side of said combustion chamber means, said burner means incorporating a plurality of inlet orifices through which a fuel/air mixture can enter said combustion chamber means; a plurality of outlet passages through which the product of combustion of said mixture when ignited can leave said combustion chamber means, said outlet passages corresponding in number to the number of said plurality of inlet orifices in said burner means and said inlet orifices and said outlet passages being disposed within a unit formed from two identical extruded blocks of heat conducting material each extruded block having a plurality of grooves therein such that when the blocks are arranged together in face to face relationship the grooves of each block define together said inlet orifices and said outlet passages, said combustion chamber means and said outlet passages being defined entirely by a core of heat conducting material, said burner means and said combustion chamber means being arranged so that flames formed by ignition of said fuel/air mixture will be held at positions from which the major portion of the heat arising from combustion of said fuel/air mixture will be conducted through said core to said fluid.
 4. Heating apparatus as claimed in claim 3 wherein said combustion chamber means is formed by removing material from each block to form a recess, said recess having a depth greater than the depth of said grooves.
 5. Heating apparatus for imparting heat to a fluid comprising: combustion chamber means; burner means at one side of said combustion chamber means, said burner means incorporating a plurality of inlet orifices through which a fuel/air mixture can enter said combustion chamber means; a plurality of outlet passages through which the products of combustion of said mixture when ignited can leave said combustion chamber means, said plurality of outlet passages corresponding in number to the number of said inlet orifices in said burner means, the dimensions of each of said inlet orifices measured at right angles to the direction of flow of said fuel/air mixture being equal to the dimensions of each of said outlet passages measured in a direction at right angles to the direction of flow of said product of combustion, said combustion chamber means and said outlet passages being defined entirely by a core of heat conducting material, said burner means and said combustion chamber means being arranged so that flames formed by ignition of said fuel/air mixture will be held at positions from which the major portion of the heat arising from combustion of said fuel/air mixture will be conducted through said core to said fluid. 